TEMPLATE
TEMPLATE\CONTROLTEMPLATES
TEMPLATE\XML

using Microsoft.SharePoint;
using Microsoft.SharePoint.WebControls;

namespace SymbolFieldType
{
    public class SymbolField : SPFieldMultiLineText
    {
        public SymbolField (SPFieldCollection fields, string fieldName)
            : base(fields, fieldName)
        {
            ;
        }

        public SymbolField (SPFieldCollection fields, string typeName, string displayName)
            : base(fields, typeName, displayName)
        {
            ;
        }

        public override Microsoft.SharePoint.WebControls.BaseFieldControl FieldRenderingControl
        {
            get
            {
                BaseFieldControl control = new SymbolFieldControl();
                control.FieldName = this.InternalName;
                return control;
            }
        }

        public override string GetValidatedString (object value)
        {
            if (this.Required || value.ToString().Equals(string.Empty))
            {
                throw new SPFieldValidationException("Symbols not assigned");
            }
            return base.GetValidatedString(value);
        }

    }
}

using System.Configuration;
using System.Text;
using System.Web.UI.WebControls;
using Microsoft.SharePoint.WebControls;

namespace SymbolFieldType
{
    public class SymbolFieldControl : BaseFieldControl
    {
        #region [ Protected properties          ]

        protected ListBox symbolListBox;

        #endregion

        #region [ Overridden methods            ]
        
        protected override string DefaultTemplateName
        {
            get
            {
                return @"SymbolFieldControl";
            }
        }

        public override object Value
        {
            get
            {
                this.EnsureChildControls();
                return GetSelectedValues(this.symbolListBox);
            }
            set
            {
                this.EnsureChildControls();
                string selectedItemsString = (string)this.ItemFieldValue;
                foreach (ListItem i in symbolListBox.Items)
                {
                    if (i.Value.Length > 0 && selectedItemsString.Contains(i.Value))
                    {
                        i.Selected = true;
                    }
                }
            }
        }

        protected override void CreateChildControls ()
        {
            if (this.Field == null || this.ControlMode == SPControlMode.Display)
                return;
            base.CreateChildControls();

            this.symbolListBox = (ListBox)TemplateContainer.FindControl("symbolListBox");

            if (this.symbolListBox == null)
                throw new ConfigurationException("SymbolFieldControl.ascx is corrupt.");

            if (!this.Page.IsPostBack)
            {
                this.symbolListBox.Items.AddRange(new ListItem[]{ 
                  new ListItem(string.Empty, null), 
                  new ListItem("Ätzend", "Aetzend.png"), 
                  new ListItem("Brandfördernd", "Brandfoerdernd.png"), 
                  new ListItem("Explosiv", "Explosiv.png"), 
                  new ListItem("Flammbar 1", "Flamm1.png"),
                  new ListItem("Flammbar 2", "Flamm2.png"),
                  new ListItem("Flammbar 3", "Flamm3.png"),
                  new ListItem("Giftig", "Giftig.png"),
                  new ListItem("Leicht entzündlich", "LeichtEntzuendlich.png"),
                  new ListItem("Reizend", "Reizend.png"),
                });
                
                this.symbolListBox.SelectionMode = ListSelectionMode.Multiple;
            }
        }

        #endregion

        #region [ Private methods               ]

        private string GetSelectedValues (ListBox listBox)
        {
            StringBuilder selectedItems = new StringBuilder();
            foreach (ListItem item in listBox.Items)
            {
                if (item.Selected)
                {
                    selectedItems.Append(");
                    selectedItems.Append(item.Value);
                    selectedItems.Append("\">");
                }
            }
            return selectedItems.ToString();
        }

        #endregion
    }
}

<%@ Control Language="C#" Debug="true" %>
<%@ Assembly Name="Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>
<%@ Register TagPrefix="SharePoint" Assembly="Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c"
             Namespace="Microsoft.SharePoint.WebControls" %>

<SharePoint:RenderingTemplate ID="SymbolFieldControl" runat="server">
  <Template>
    <asp:ListBox ID="symbolListBox" runat="server"  />
  Template>
SharePoint:RenderingTemplate>

<asp:ListBox ID="symbolListBox" runat="server"  />

protected ListBox symbolListBox;

xml version="1.0" encoding="utf-8" ?>
<FieldTypes>
    <FieldType>
        <Field Name="TypeName">SymbolSelectionField>
        <Field Name="ParentType">NoteField>
        <Field Name="SQLType">ntextField>
        <Field Name="TypeDisplayName">Danger signField>
        <Field Name="TypeShortDescription">Custom symbol columnField>
        <Field Name="Sortable">FALSEField>
        <Field Name="Filterable">FALSEField>
        <Field Name="UserCreatable">TRUEField>
        <Field Name="ShowInListCreate">TRUEField>
        <Field Name="ShowInSurveyCreate">TRUEField>
        <Field Name="ShowInDocumentLibraryCreate">TRUEField>
        <Field Name="ShowInColumnTemplateCreate">TRUEField>
        <Field Name="FieldTypeClass">SymbolFieldType.SymbolField, SymbolFieldType, Version=1.0.0.0, Culture=neutral, PublicKeyToken=!Hier den eigenen Token einsetzen!Field>
        <RenderPattern Name="DisplayPattern">
            <Switch>
                <Expr>
                    <Column/>
                Expr>
                <Case Value="">
                Case>
                <Default>
                    <Column/>
                Default>
            Switch>
        RenderPattern>
    FieldType>
FieldTypes>

<Default>
    <Column/>
Default>